1. Field of the Invention
This invention relates to the regeneration of scale-coated activated carbon. More particularly, an acid wash method is disclosed for removing mineral scale formed on activated carbon from the treatment of crude sodium carbonate solutions.
2. Description of the Prior Art
In the manufacture of soda ash (sodium carbonate) from trona ore, crude sodium carbonate process solutions are purified by various unit operations to remove inorganic and organic impurities. Activated carbon has been employed in this application to adsorb organic impurities from crude sodium carbonate solutions, as is described by Hellmers et al, U.S. Pat. No. 3,260,567, and Ilardi et al, U.S. Pat. No. 3,933,977. The activated carbon is typically contained in packed bed columns, through which the crude sodium carbonate process stream is passed. The carbon is periodically reactivated by washing with hot water or thermally, by contacting the spent carbon with steam, to regenerate the carbon adsorptivity.
A serious drawback associated with the use of activated carbon beds in this particular application is the gradual accumulation of inorganic mineral scale of heretofore unidentified composition on the carbon. Formation of the inorganic mineral scale on the activated carbon appears to be exacerbated with crude sodium carbonate solutions which are highly saturated with carbonate as well as calcium, silica, alumina, magnesium and other mineral components associated with trona ore.
A serious, undesirable consequence of the mineral scale formation on the activated carbon particles is the decrease in carbon activity resulting from the loss of adsorptive capacity of the carbon. In severe instances of such scaling, the activated carbon bed literally becomes cemented into a single mass by the scale, thereby impeding regeneration of the carbon.
Conventional methods of regenerating spend activated carbon are ineffective for carbon coated with such mineral scale. Periodic hot water washes or thermal regeneration with steam have been found ineffective for removing the inorganic scale from activated carbon and for regenerating the activity of the scale-coated carbon to an adequate level of carbon adsorptivity.
Coglaiti et al, in U.S. Pat. No. 3,528,766, recognize that a hot water wash alone is inadequate for reactivating scale-coated carbon used to treat sodium carbonate solution and suggest that a simple muriatic acid (HCl) rinse is sufficient for removing the mineral scale from activated carbon beds. Despite this disclosure by Coglaiti et al, washing with hydrochloric acid alone has not been found to be totally effective for removing scale from activated carbon which has been used to treat crude sodium carbonate solutions during the manufacture of soda ash and for regenerating spent carbon activity to a satisfactory level of adsorptivity.
Although hydrochloric acid washes may provide marginal regeneration for spent carbon with minor scaling, activated carbon used to treat crude sodium carbonate solutions over extended periods of time typically suffers a significant decline in its carbon activity that is not susceptible to regeneration even with periodic hydrochloric acid or hot water washes.
As a consequence, the scale-coated activated carbon must ultimately be discarded when scale deposition results in a severe loss of carbon adsorptivity.